[PATCH] arm64: mm: assign PXN permission in trans_pgd

[Catalin Marinas]

On Thu, Nov 11, 2021 at 02:03:16PM +0800, Pingfan Liu wrote:
> trans_pgd_create_copy() can hit "VM_BUG_ON(mm != &init_mm)" in the
> function pmd_populate_kernel().

I think that's more important to fix than the PXN. You may want to
change the subject to something like "Fix VM_BUG_ON ..."

> p?d_populate() helpers resort to the input parameter mm_struct to decide
> the attribute: UXN or PXN. And plus the fact, either hibernation or
> kexec runs in privilege mode. So the bug can be fixed by assigning
> &init_mm to the callsites of these helpers.

That's the table PXN, so it's not a big deal if we miss it.

> Signed-off-by: Pingfan Liu <kernelfans at gmail.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Pasha Tatashin <pasha.tatashin at soleen.com>
> Cc: James Morse <james.morse at arm.com>
> Cc: Matthias Brugger <mbrugger at suse.com>
> To: linux-arm-kernel at lists.infradead.org

A Fixes: tag would be nice, we just need to decide which.

> ---
>  arch/arm64/mm/trans_pgd.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> index d7da8ca40d2e..7b43d179e94d 100644
> --- a/arch/arm64/mm/trans_pgd.c
> +++ b/arch/arm64/mm/trans_pgd.c
> @@ -67,7 +67,7 @@ static int copy_pte(struct trans_pgd_info *info, pmd_t *dst_pmdp,
>  	dst_ptep = trans_alloc(info);
>  	if (!dst_ptep)
>  		return -ENOMEM;
> -	pmd_populate_kernel(NULL, dst_pmdp, dst_ptep);
> +	pmd_populate_kernel(&init_mm, dst_pmdp, dst_ptep);
>  	dst_ptep = pte_offset_kernel(dst_pmdp, start);

Commit 5de59884ac0e ("arm64: trans_pgd: pass NULL instead of init_mm to
*_populate functions") actually replaced &init_mm with NULL. Commit
59511cfd08f3 ("arm64: mm: use XN table mapping attributes for
user/kernel mappings") introduced the VM_BUG_ON.

So we either revert Pasha's trans_pgd commit (as per your patch) or we
fix Ard's commit to make it aware of a NULL mm and either skip the table
UXN altogether or treat it as init_mm.

-- 
Catalin